A Forest Management Non point Source Risk Assessment 



disagreement over the relative risk values, on a 

 simple scale of 1 to 5, to be used in the body of 

 the matrix as well as the basic combinations of 

 slope and erodibility that would produce differ- 

 ent potential impacts. 



We convened the group of experts four 

 times and, through the Nominal Group Tech- 

 nique, arrived at the consensus matrix appear- 

 ing in Figure I-l. The horizontal axis is com- 

 posed of 4 principal forest management prac- 

 tices: (1) clearcutting, (2) partial cutting, 

 (3) site preparation, and (4) road construction. 

 Each principal practice is further split into spe- 

 cific methods, treatments, or designs. 



The group decided that four slope classes on 

 the vertical axis were sufficient and distinctly 

 different in inherent erosion risk, and that three 

 basic soil erodibility classes were also ade- 

 quate. These classes were based primarily on 

 the geologic erosion factors described in the 

 R1-R4 Sediment Yield Prediction Procedure 

 (USDA Forest Service 1981). Everyone agreed 

 that alluvial and granitic soils were highly erod- 

 ible, those formed from Belt Series metamorph- 

 ics had low erodibility, and that most other 

 substrates fell into the moderate erodibility class. 



Howard Creek 



The Howard Creek watershed, totalling 

 501 5 hectares, is located approximately 32 kilo- 

 meters southwestof Missoula, Montana. Eleva- 

 tion in the watershed ranges from 1 190 to 1770 

 meters. Average annual precipitation at mean 

 elevation is about 100 cm, and roughly 60 

 percent of that falls as snow. The forest cover is 

 primarily Douglas-fir mixed with sub-alpine fir 

 at the higher elevations and Ponderosa pine at 

 the lower elevations and south aspects. The 

 main stem of Howard Creek is oriented east- 

 west while the three sub-basins. Tepee Creek, 

 North Fork of Howard Creek, and Krystal Creek, 



are oriented north-south. (See Figure 1-2.) This 

 map, and the following maps were prepared 

 using PAMAP (1989) CIS software and infor- 

 mation hand-digitized at a scale of 1:24000. 



The Lolo National Forest Land Systems 

 Inventory maps the primary geologic groups 

 found in Howard Creek as metasedimentary 

 and undifferentiated materials. (See Figure I- 

 3.) 



Metasedimentary parent materials are de- 

 rived from Belt S uper Group quartzite , argillite, 

 and siltite. Rock fragment hardness is variable 

 depending upon the degree of rock weathering. 

 Weathering is dependent on associated faults, 

 preponderance of argillites, and calcium car- 

 bonate content. These materials were classified 

 as either L-low or M-moderate erodibility for 

 the erosion-impact matrix. 



Undifferentiated geology is composed of 

 materials derived from Belt Super Group 

 metasedimentary rocks or weakly weathered 

 granitic rocks. Materials include alluvium on 

 terraces and flood plains; shallow soils on flood 

 scoured foot slopes and stream breaklands, 

 strongly frost churned broadly convex ridges, 

 and glacial outwash on plains. These are classi- 

 fied as "H" — highly erodible in the Montana 

 erosion-impact matrix. 



Like much of western Montana, the water- 

 shed has a "checkerboard" pattern of land own- 

 ership. Champion Timberlands owns 34 per- 

 cent of the watershed. Plum Creek Timber owns 

 23 percent of the watershed, and the remaining 

 42 percent is managed by the Lolo National 

 Forest. Ownership was mapped by the GIS and 

 is shown in Figure 1-4. Timber harvest-related 

 activities between 1981 and 1986 impacted 17 

 percent of the watershed. The location of these 

 activities is shown in Figure 1-5. In a report to 

 the Lolo Forest Supervisor (Munther and others 

 1987), cumulative watershed effects in Howard 

 Creek were estimated to have produced a sedi- 

 ment load increase of 50 percent and water yield 



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Flathead Basin Cooperative Program Final Report 



